There is an intimate relationship between cannon chamber volume and propellant performance. In an artillery cannon that fires multiple zones, it is important that complete burning occur and pressures remain manageable at all firing zones. In a fixed volume chamber cannon, complete burning and manageable pressure usually cannot be achieved over the desired range of operation simply by varying the amount of the same type of propellant. In the past, cannon developers addressed this problem by using different types and/or configurations of propellant for shots at different zones. Theoretically, a cannon with a variable volume chamber offers increased flexibility to the cannon and propellant developer by removing the constraint associated with fixed volume. Further, a variable volume chamber theoretically provides an opportunity to more easily adapt propellant from one cannon to another.
A well designed variable volume chamber cannon can deliver different muzzle exit velocities to the projectile either by changing the chamber volume or by changing the amount of propellant in the chamber. The ability to vary muzzle velocity allows for improved zoning characteristics and greater Multiple Round Simultaneous Impact (MRSI) capability.
A design for a variable volume chamber cannon is documented in U.S. Pat. No. 6,571,676 to Folsom et al., and describes chambers of volumes situated on a turntable. The operator selects the appropriate chamber for firing. Alternatively there have been discussions regarding cannons with peripheral chambers adjacent to the primary chamber with valves in between. Under this concept it may be possible to increase the effective chamber volume by opening valves between the primary chamber and the peripheral chamber to accommodate the maximum amount of propellant, but this method does not allow for the reduction of the primary chamber volume for smaller amounts of propellant. The Folsom device has not proved to be practical in the field. Thus there remains a need for a cannon with an adjustable chamber volume so as to provide the developer and user with greater flexibility.